Evaluation of chemical cleaning efficiency of organic-fouled SWRO membrane by analyzing filtration resistance

Membrane fouling is an unavoidable phenomenon in the operation of seawater reverse osmosis and a major obstacle to economic and efficient operation. In particular, membrane fouling by organic matter negatively affects productivity, product quality, and process cost. Therefore, a chemical cleaning process is essential to prevent interruptions for an effective RO membrane filtration process. Firstly, this study focused on the proper chemical cleaning condition for commercial polyamide RO membranes purchased from two companies. The flux decline rate of SWC5+ membrane was higher than that of SW39HRLE400 membrane regardless of organic foulants because the initial zeta potential of SWC5+ membrane (-21.17mV) was lower than that of SW30HRLE400 membrane (-30.11mV) and the repulsive force between membrane surface and foulants was also lower. In addition, we attempted to evaluate cleaning efficiency according to the chemical cleaning conditions and investigate the cause of fouling by analyzing membrane resistance fraction after cleaning the organic-fouled membrane. As a result of cleaning in place (CIP), as the concentration and cleaning time increased when NaOH, trisodium phosphate 12, sodium tripolyphosphate, and ethylenediaminetetraacetic acid-4Na were used without sodium dodecylsulfate, cleaning efficiency increased significantly. Subsequently, in the condition of CIP using NaOH at pH 11 for 60min, the cleaning efficiency of membrane fouled by humic acid was higher that of membrane fouled by sodium alginate (SA) due to the strong interaction between hydrophobic membrane and hydrophobic foulants in the case of both membranes. To find the fouling rate and fouling cause, total resistance (R-t) was fractionated by the initial membrane resistance (R-m), resistance removed by flushing (R-c), resistance removed by CIP (R-a), and irreversible resistance (R-i). R-c and R-a in both membranes showed a similar proportion in R-t. The cause of fouling of the membrane fouled by SA was mainly the absorption of membrane surface, and that of membrane fouled by HA was the absorption of membrane surface and pore blocking by cake layer.